An enhanced heat transfer tube with discrete bidirectionally inclined ribs

ABSTRACT

An enhanced heat transfer tube with discrete bidirectionally inclined ribs (DBIR-tube) provides enhanced convective heat transfer both in laminar and turbulent flow regions. The inner surface of the tube is formed with bidirectionally inclined ribs, including left hand inclined ribs and right hand inclined ribs, with the ribs being in the form of discrete strip-like protrusions which respectively form a certain angle with respect to the axis of the tube and incline in two directions. With the hydrodynamic inner diameter of the tube represented by “d”, each bidirectionally inclined rib has a height less than or equal to 0.2 d, a circumferential width less than or equal to 0.5 d, and an axial length less than or equal to 2 d. A certain angle formed between the axis of each rib and the axis of the tube is ±5° to ±85°, the positive sign meaning that the internal rib is orientated in a right hand declining direction, and the negative sign meaning that the internal rib is orientated in a left hand declining direction.

TECHNICAL FIELD

The invention relates to an enhanced heat transfer tube with discretebidirectionally inclined ribs, which belongs to the field of enhancedheat transfer and heat exchanger.

BACKGROUND ART

Shell-and-tube type heat exchangers have extensively and greatlyapplication in areas such as petroleum engineering, chemicalengineering, and power engineering. Smooth circular tubes, which havethe advantages of easy manufacturing, high reliability and low cost, aregenerally used in shell-and-tube type heat exchangers. However, ordinaryshell and tube heat exchangers using circular tubes are bulky andmaterial-cost, due to their weak heat transfer. In order to overcomethese shortages, a variety of enhanced heat transfer tubes have beenadopted for substituting smooth circular tubes. Various enhancedconvective heat transfer tubes have been invented in recent 30 yearsbased on wall surface disturbing enhancement techniques. Especially,rough surface enhanced tubes, such as spirally grooved tubes, transversegrooved tubes and micro finned tubes, made by rolling have beensuccessfully and widely used in engineering. In addition, inserts, suchas twisted-tape inserts and spring inserts, also have lots ofapplications. Most of the existing enhanced tubes suffer from highflowing resistance or pressure drop, fouling (dust accumulating) in theback-flow areas near the grooves or ribs, as well as low efficiency andhigh cost in tube manufacturing.

DISCLOSURE OF THE INVENTION

An object of the invention is to solve the problems found from the priorart by provide a new type of enhanced heat transfer tube, named byenhanced heat transfer tube with discrete bidirectionally inclined ribs(for simplification, the tube is called as “DBIR-tube” bellow), whichincludes a plurality of bidirectionally inclined ribs on the tube wall,with the ribs being in the form of discrete strip-like protrusions whichrespectively form a certain angle with respect to the axis of the tubeand incline in two directions. Longitudinal vortex flows, especiallynear the tube wall, can be induced by the bidirectionally inclined ribsdisposed periodically on the tube wall. As a result, the heat transferis significantly enhanced. The DBIR-tube of the present invention ismade on the basis of the Field Synergy Principle of convective heattransfer (c.f. Guo Z. Y, Mechanism and Control of Convective HeatTransfer-Coordination of Velocity and Heat Flow Field, Chinese ScienceBulletin, 46(7): 596-599 April 2001.). An enhanced heat transfer isobtained from the improvement of the synergy between the velocity fieldand the temperature gradient field with less additional pressure drop.Theoretical analysis based on Field Synergy Principle shows that thelongitudinal vortex flow is the most effective way for enhancing heattransfer. The invented DBIR-tube has better heat transfer performanceand less friction loss in turbulent regions and transition regions thantraditional spirally grooved tubes, transverse grooved tubes, spirallyfinned tubes and rough finned tubes. As a result, it can overcome thedisadvantages of the existing enhancing techniques.

According to the present invention, an enhanced heat transfer tube withdiscrete bidirectionally inclined ribs (DBIR-tube) is provided, whereinthe inner surface of the tube is formed with bidirectionally inclinedribs including left hand inclined ribs and right hand inclined ribs,with the ribs being in the form of discrete strip-like protrusions whichrespectively form a certain angle with respect to the axis of the tubeand incline in two directions, and wherein, if the hydrodynamic innerdiameter of the tube is represented by “d”, each inclined rib has aheight less than or equal to 0.2 d, a circumferential width less than orequal to 0.5 d, and an axial length less than or equal to 2 d.

Preferably, a certain angle formed between the axis of each rib and theaxis of the tube is ±5° to ±85°, wherein the positive sign means thatthe internal rib is orientated in a right hand declining direction, andthe negative sign means that the internal rib is orientated in a lefthand declining direction.

Preferably, the shape of the cross section of the rib is in the form ofone or combined more of circular arch, rectangle, triangle, circularsector, streamline, and any combination of curve line and straight line.

Preferably, the axis of the rib is in the form of one or combined moreof straight line, zigzag line, arch line, spiral line, and curve line.

Preferably, the inner surface of the tube is in the form of one orcombined more of smooth surface, spiral rib surface, and low ribsurface.

Preferably, the outer surface of the tube is in the form of one orcombined more of grooved surface, smooth surface, spiral rib surface,low rib surface, and finned surface.

Compared with the existing techniques, the present invention has thevirtues of significant enhanced heat transfer, low additional pressureloss and simple manufacturing process. In the turbulent flow region, theheat transfer coefficient of the DBIR-tube is 80% to 150% higher thanthat of the smooth tube, and 30% higher than that of the transversegrooved tube (one best enhanced tube), while the pressure loss is 20% to50% lower than that of the transverse grooved tube. With regard to theconvection heat transfer happened in transition regions, the heattransfer effect is also greatly improved. Compared with the transversegrooved tube, the DBIR-tube is of good anti-fouling performance becausethere is no back flow (transverse vortex flow) formed near thebidirectionally inclined ribs and there is no stagnant area formedinside the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the structure of a DBIR-tube of thepresent invention.

FIG. 2 is a sectional view taken along line A-A of FIG. 1.

FIG. 3 is an enlargement view of part B of FIG. 2.

FIG. 4 is a circumferentially part-deployed schematic view of anotherDBIR-tube of the present invention.

FIG. 5 is a circumferentially part-deployed schematic view of stillanother DBIR-tube of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The enhanced heat transfer tube with discrete bidirectionally inclinedribs (DBIR-tube) of the present invention comprises a plurality ofbidirectionally inclined ribs on the inner surface of the tube wall,with the ribs being in the form of discrete strip-like protrusions whichrespectively form a certain angle with respect to the axis of the tubeand incline in two directions. Generally, the height of eachbidirectionally inclined rib is no more than (less than or equal to) 0.2d, the circumferential width of it is no more than 0.5 d, and the axiallength of it is no more than 2 d, wherein d is the hydrodynamic innerdiameter of the tube. The term “discrete” used herein meansdiscontinued, which differs from the case in spirally grooved tubes(continuously spiral), spirally finned tubes (continuously spiral), andtransverse tubes (circumferential continuously). Meanwhile, the“bidirectionally inclined rib” refers to a non-even or rough surfaceelement (strip-like protrusion) having a certain length. The DBIR-tubescan be made by mould pressing or rolling of ordinary smooth tubes, lowfinned tubes, and spirally finned tubes, etc., and it can also be madeduring the process of forming seamless tubes by rolling or the processof forming seamed tubes by weld-jointing. Due to the plurality ofdiscrete bidirectionally inclined ribs, longitudinal vortex flows and/orother secondary flows will be induced in the invented DBIR-tube, and thevortex flows and secondary flows are mainly appeared near the tube wall,so that the turbulent flow heat transfer in the turbulent regions andthe convective heat transfer in the transition regions are enhanced.

The First Embodiment

FIG. 1 shows the structure of a DBIR-tube of the present invention. Theinner surface of the tube is provided with a plurality of discrete,two-direction helical protrusions (named as internal discrete doublehelical ribs or internal discrete bidirectionally inclined ribs). Theouter surface of the tube is provided with a plurality of discretedouble helical grooves.

In FIGS. 1 and 2, reference numeral 1 designates the discrete doublehelical ribs, and reference numeral 2 designates the discrete doublehelical grooves, with the helical ribs and the corresponding helicalgrooves being formed simultaneously during processing. In FIG. 1,reference sign “d” designates the hydrodynamic inner diameter of thetube, reference sign “P” designates the axial length of each internalinclined rib, and reference sign “C” designates the helical angle of theinternal inclined ribs. In FIG. 3, reference sign “h” designates theheight of the internal inclined ribs. The tubes of the invention meetthe condition of P=0.3 d, h=0.05 d, and C≈±45°, wherein the positivesign (+) means that the internal rib is orientated in a right handdeclining direction, and the negative sign (−) means that the internalrib is orientated in a left hand declining direction.

The Second Embodiment

FIG. 4 shows the structure of another DBIR-tube of the present inventionin a circumferentially part-deployed schematic view. The inner surfaceof the tube is provided with a plurality of discrete, two-directionhelical protrusions (discrete bidirectionally inclined ribs or discretedouble inclined ribs). The outer surface of the tube is smooth, withoutany protrusion or groove.

In FIG. 4, reference numeral 3 designates the discrete bidirectionallyinclined ribs, which are symmetrical disposed. Each pair of inclinedribs, composed of an inclined rib which is orientated in a right handdeclining direction and a circumferentially neighboured inclined ribwhich is orientated in a left hand declining direction, form a vortexgenerator. In each cross-section of the tube, there are two vortexgenerators formed by four inclined ribs. In FIG. 4, reference sign “C”designates the helical angle of the internal inclined ribs, and C≈±50°,wherein the positive sign (+) means that the internal rib is orientatedin a right hand declining direction, and the negative sign (−) meansthat the internal rib is orientated in a left hand declining direction.Streamlines designated by reference numeral 4 schematically show thelongitudinal vortex flows generated near the tube wall. Since thelongitudinal vortex flows are mainly created near the tube wall, heattransfer may be enhanced in both laminar and turbulent flow regions. Thediscrete inclined ribs nearly do not create any transverse vortex flowin the fluid flowing through the tube, and do not significantly reducethe flowing area of the tube. As a result, the flowing resistance orpressure loss is much lower than that of traditional spirally groovedtubes, transverse grooved tubes and spirally finned tubes.

The Third Embodiment

FIG. 5 shows the structure of yet another DBIR-tube of the presentinvention in a circumferentially part-deployed schematic view. The innersurface of the tube is provided with a plurality of discrete,two-direction helical protrusions (bidirectionally inclined ribs). Theouter surface of the tube is provided with a plurality of discretedouble helical grooves.

In FIG. 5, reference numeral 5 designates the discrete double helicalribs which are asymmetrically arranged, and reference numeral 6designates the discrete double helical grooves which are alsoasymmetrically arranged. On the inner surface, each pair of inclinedribs, composed of an inclined rib which is orientated in a right handdeclining direction and a circumferentially neighboured inclined ribwhich is orientated in a left hand declining direction, form a vortexgenerator. Within a small circumferential segment of the tube, which hasan axial length of less than 0.5 d, there are three vortex generatorsformed by six inclined ribs.

Preferred manufacturing methods of the DBIR-tube include rolling ormould pressing. A preferred rolling process of the DBIR-tube withbidirectionally inclined helical ribs on the inner surface andbidirectionally inclined helical grooves on the outer surface is carriedout in the following way. Specifically, a tube is formed by processingrollers, the forming surfaces of which are provided with discretebidirectionally inclined low-profile protrusions; when the tube isformed by rolling, discrete helical bidirectionally inclined grooves areformed on the outer surface under the extrusion forces of thelow-profile protrusions, and in the meantime, discrete bidirectionallyinclined ribs are formed on the inner surface. On the other hand, twomanufacturing processes are preferred for forming the DBIR-tube withdiscrete helical bidirectionally inclined ribs on the inner surface andbeing smooth on the outer surface. The first one is similar to that ofthe traditional enhanced heat transfer tube with inner spirally fins andbeing smooth on the outer surface, and the second one is furtherprocessing (for example, cold drawing) the DBIR-tube with helicalbidirectionally inclined ribs on the inner surface and helicalbidirectionally inclined grooves on the outer surface obtainedpreviously by rolling or mould pressing as discussed above. As a resultof the discontinuousness of the bidirectionally inclined ribs, themanufacturing efficiency of the rolling or mould pressing process ofDBIR-tube is several times higher than that of the spirally finnedtubes, the transverse grooved tubes and the spirally finned tubes.Therefore, the manufacturing cost is greatly reduced.

1. An enhanced heat transfer tube with discrete bidirectionally inclinedribs, wherein the inner surface of the tube is formed withbidirectionally inclined ribs including left hand inclined ribs andright hand inclined ribs, with the ribs being in the form of discretestrip-like protrusions which respectively form a certain angle withrespect to the axis of the tube and incline in two directions, andwherein, if the hydrodynamic inner diameter of the tube is representedby “d”, each bidirectionally inclined rib has a height less than orequal to 0.2 d, a circumferential width less than or equal to 0.5 d, andan axial length less than or equal to 2 d.
 2. The enhanced heat transfertube with discrete bidirectionally inclined ribs of claim 1, wherein acertain angle formed between the axis of each rib and the axis of thetube is 5° to ±85°, wherein the positive sign means that the internalrib is orientated in a right hand declining direction, and the negativesign means that the internal rib is orientated in a left hand decliningdirection.
 3. The enhanced heat transfer tube with discretebidirectionally inclined ribs of claim 1, wherein the shape of the crosssection of the rib is in the form of one or combined more of circulararch, rectangle, triangle, circular sector, streamline, and anycombination of curve line and straight line.
 4. The enhanced heattransfer tube with discrete bidirectionally inclined ribs of claim 1,wherein the axis of the rib is in the form of one or combined more ofstraight line, zigzag line, arch line, spiral line, and curve line. 5.The enhanced heat transfer tube with discrete bidirectionally inclinedribs of claim 1, wherein the inner surface of the tube is in the form ofone or combined more of smooth surface, spiral rib surface, and low ribsurface.
 6. The enhanced heat transfer tube with discretebidirectionally inclined ribs of claim 1, wherein the outer surface ofthe tube is in the form of one or combined more of grooved surface,smooth surface, spiral rib surface, low rib surface, and finned surface.